1. Field of the Invention
The present invention relates to an optical disk device and a tilt correction method able to correct an incident angle of a laser beam so that the laser beam perpendicularly irradiates the surface of an optical disk.
2. Description of the Related Art
Optical disks, such as CD-ROMs, CD-Rs, and CD-RWs, are widely used. Particularly, in recent years and continuing, DVD type optical disks show up in greater numbers, and have higher track density and thus higher capacity than the above CD type optical disks. The reproduction-only DVD has become wide-spread, and the recordable DVD is being brought into practical use.
To record or reproduce data on an optical disk, an object lens is provided to condense a laser beam to form a small light spot on a track of the optical disk. If the optical axis of the object lens tilts relative to the optical disk, optical aberrations arise at the light spot, and this may present obstacles to data recording and reproduction. Below, the angle between the optical axis of the object lens and the normal line to a recording surface of the optical disk is called as “tilt of the optical system” or just “tilt”.
Along with the higher and higher density of the above optical disks, there appear more and more apparatuses equipped with mechanisms for correcting the tilt that causes or increases the optical aberrations.
In the related art, a tilt correction device has been proposed, which includes a light emission element using LEDs (light emission diode), a tilt sensor formed by a two-division light detector for detecting the light emitted from the light emission element and reflected at the optical disk, and a device for tilting an object lens or an optical pick-up based on the output of the tilt sensor.
In the above tilt correction device, the arrangement of the tilt sensor is adjusted so that the two parts of the light detector in the tilt sensor yield the same outputs when detecting an optical disk originally installed, that is, an optical disk free from tilt. The light emitted from the light emission element is reflected by the optical disk, enters into the tilt sensor, and forms a light spot on the tilt sensor. By extracting a differential signal with the two-division detector in the tilt sensor, tilt can be detected.
In addition to the above method, there are other methods for detecting tilt. For example, a tilt correction value causing the RF signal and the TE (tracking error) signal to become the maximum can be extracted as tilt.
Another method for correcting tilt involves measuring in advance the tilt at a number of positions on the optical disk, and making tilt corrections based on the measured data. Another method involves measuring the tilt of the optical disk continuously and performing tilt servo.
Such a technique is disclosed in Japanese Patent Application Laid Open No. 2000-339727, which proposes an optical disk device to measure tilt at a number of positions in the radial direction of the optical disk and store the measured tilt data in advance, and correct the tilt based on the stored tilt data.
However, in the method of measuring in advance tilt data, which cause the RF signal and the TE signal to become the maximum, at a number of positions, and making tilt corrections based on the measured tilt data, because the magnitude of the tilt and its variation are small in the inner region of the optical disk, and large in the peripheral region of the optical disk, complicated calculations and tilt data of many positions are needed for interpolation to correct tilt between the two regions.